Hi santo the gears are a straight slip in but both main and pinion are required for the change. Personally on a 50 size reel the slower gearing is actually better and much stronger as well.
The first model grand waves 40 and 50 were the slower 4.9.1 nothing g wrong with them, the same with the Daiwa slower SG50H same reel as the sl50sh just slower and had a stronger cast spool and 2 less bearings, this is an awesome reel but was discontinued in favour of the better selling sl50sh at 6.1. The slower gears on the SG also tank tough. Same goes for the 15/30 ; 20/40 same gearing and stronger than the 6.1 geared speed master, see a pattern forming Now?
For larger fish and stronger fish and boot fishing also the slower ratio is preferred above speed as it permits more torque. The gear teeth on the slower model both main and pinion are lots stronger than the 6.1 which if finer cut to up the speed.
Hope this answers your request.